Historically, a steel cord, a tyre bead wire or the like has generally been produced through the processes of: hot-rolling a high-carbon steel containing about 0.7 to 0.8% carbon (corresponding to JIS G3502 (SWRS72A and SWRS82A)); thereafter producing a steel wire rod about 5.0 to 6.4 mm in diameter by controlling the cooling conditions thereof; successively subjecting it to primary wire drawing, patenting treatment, secondary wire drawing, (secondary patenting treatment in the case of a steel cord), Cu—Zn dual phase plating and blueing treatment; and then finally applying wet wire drawing (finish wire drawing) and resultantly obtaining a prescribed wire diameter. Among the above processes, the patenting treatment (annealing treatment) is applied in order to obtain a fine pearlite structure that is beneficial to wire drawability. But, with the aim of improving productivity, enhancing energy saving, and reducing costs, promoted has been the development of a hot-rolled wire rod (direct-patenting wire rod) capable of omitting heat treatment such as patenting or the like.
For example, Patent document 1 (JP-B No. 60900/1991) proposes a wire rod defined by specifying the relationship among the carbon equivalent, tensile strength and coarse pearlite percentage of a high-carbon steel wire rod as a steel wire rod being excellent in drawing die service life and having a low frequency of wire breakage (refer to CLAIMS, from the line 19 in the first paragraph to the line 6 in the second paragraph, and from the line 7 to the line 33 in the fifth paragraph). According to the document, the average value of tensile strength is controlled in relation to a carbon equivalent particularly on the basis of the knowledge that “a direct-patenting wire rod has a certain optimum value in tensile strength and the wire breakage rate increases when the tensile strength is either lower or higher than the optimum value.” As a result of the studies by the present inventors however, it has been clarified that there are cases where wire breakage cannot sufficiently be prevented from occurring during wire drawing even when such control measures are taken. Generally, in the case of a usual rolled wire rod, the mechanical properties thereof vary along the length (longitudinal portions), and the portions high in tensile strength and reduction of area and those low in tensile strength and reduction of area coexist therein. Hence, the portions locally high in strength or low in ductility are insufficiently controlled merely by regulating the average value of tensile strength as stipulated in the document, and the portions act as the origins of breakage during wire drawing and cause wire breakage.
Further, though it is not intended to propose a direct-patenting wire rod, as a method for enabling direct softening by slowly cooling a coil after hot-rolling, Patent document 2 (JP-A No. 179325/2001) discloses a method of controlling the cooling rate of the coil on a cooling conveyer after hot-rolling, the components of the steel material, the diameter of austenite grains at the start of the slow cooling, the wire diameter, the ring pitch, and the temperature of a slow cooling cover (refer to the paragraphs [0001], [0004], [0020] to [0026], and FIG. 1). However, the document does not include from the beginning the idea, of the present invention, that “it is essential to make a wire rod have the mechanical properties of low variation in order to provide a hot-rolled wire rod incomparably excellent in wire drawability.” Hence, similarly to the case of Patent document 1, the portions extremely low in strength or low in ductility locally are still insufficiently controlled.